Dose Calculation Errors Research Articles

Addressing the dosimetric impact of bone cement and vertebroplasty in stereotactic body radiation therapy.

Bone cement used for vertebroplasty can affect the accuracy on the dose calculation of the radiation therapy treatment. In addition the CT values of high density objects themselves can be misrepresented in kVCT images. The aim of our study is then to propose a streamlined approach for estimating the real density of cement implants used in stereotactic body radiation therapy. Several samples of cement were manufactured and irradiated in order to investigate the impact of their composition on the radiation dose. The validity of the CT conversion method for a range of photon energies was investigated, for the studied samples and on six patients. Calculations and measurements were carried out with various overridden densities and dose prediction algorithms (AXB with dose-to-medium reporting or AAA) in order to find the effective density override. Relative dose differences of several percent were found between the dose measured and calculated downstream of the implant using an ion chamber and TPS or EPID dosimetry. If the correct density is assigned to the implant, calculations can provide clinically acceptable accuracy (gamma criteria of 3%/2 mm). The use of MV imaging significantly favors the attribution of a correct equivalent density to the implants compared to the use of kVCT images. The porosity and relative density of the various studied implants vary significantly. Bone cement density estimations can be characterized using MV imaging or planar in vivo dosimetry, which could help determining whether errors in dose calculations are due to incorrect densities.

A tool for precise calculation of organ doses in voxelised geometries using GAMOS/Geant4 with a graphical user interface

Abstract Introduction: The limit of the method of calculating organ doses using voxelised phantoms with a Monte Carlo simulation code is that dose calculation errors in the boundaries of the organs are especially relevant for thin, small or complex geometries. In this report, we describe a tool that helps overcome this problem, accurately calculating organ doses by applying the “parallel geometry” utility feature of Geant4 through the GAMOS framework. Methods and methods: We have tried to simplify the use of this tool by automatically processing the different DICOM image modalities (CT, PT, ST, NM), and by including the automatic conversion of the structures found in a DICOM RTSTRUCT file into Geant4 volumes that build the parallel geometry. For Nuclear Medicine applications, the DICOM PT, ST or NM images are converted into probabilities of generation of primary particles in each voxel, and the DICOM CT images into materials and material densities. For radiotherapy treatments, the DICOM RTPlan or RTIonPlan may also be used, hence the user only needs to describe the accelerator geometry. We also provide a Graphical User Interface for ease of use by for inexperienced users in Monte Carlo. Results: We have tested the functionality of the tool with an I-131 thyroid cancer treatment, and obtained the expected energy deposition and dose differences, given that the particle source, geometry and structures are defined. Conclusions: In summary, we provide an easy-to-use tool to calculate, with high accuracy, organ doses, taking into account their exact geometry as painted by the medical personnel on a voxelised phantom.

Accuracy Evaluation of EPL and ETAR Algorithms in the Treatment Planning Systems using CIRS Thorax Phantom.

Background: It is recommended for each set of radiation data and algorithm that subtle deliberation is done regarding dose calculation accuracy. Knowing the errors in dose calculation for each treatment plan will result in an accurate estimate of the actual dose achieved by the tumor.Objective: This study aims to evaluate the equivalent path length (EPL) and equivalent tissue air ratio (ETAR) algorithms in radiation dose calculation.Material and Methods: In this experimental study, the TEC-DOC 1583 guideline was used. Measurements and calculations were obtained for each algorithm at specific points in thorax CIRS phantom for 6 and 18 MVs and results were compared.Results: In the EPL, calculations were in agreement with measurements for 27 points and differences between them ranged from 0.1% to 10.4% at 6 MV. The calculations were in agreement with measurements for 21 points and differences between them ranged from 0.4% to 13% at 18 MV. In ETAR, calculations were also in consistent with measurements for 21 points, and differences between them ranged from 0.1% to 9% at 6 MV. Moreover, for 18 MV, the calculations were in agreement with measurements for 17 points and differences between them ranged from 0% to 11%. Conclusion: For the EPL algorithm, more dose points were in consistent with acceptance criteria. The errors in the ETAR were 1% to 2% less than the EPL. The greatest calculation error occurs in low-density lung tissue with inhomogeneities or in high-density bone. Errors were larger in shallow depths. The error in higher energy was more than low energy beam.

Dosimetric study of Hounsfield number correction effect in areas influenced by contrast product in lungs case.

The aim of the study was dosimetric effect quantification of exclusive computed tomography (CT) use with an intravenous (IV) contrast agent (CA ), on dose distribution of 3D-CRT treatment plans for lung cancer. Furthermore, dosimetric advantage investigation of manually contrast-enhanced region overriding, especially the heart. Ten patients with lung cancer were considered. For each patient two planning CT sets were initially taken with and without CA. Treatment planning were optimized based on CT scans without CA. All plans were copied and recomputed on scans with CA. In addition, scans with IV contrast were copied and density correction was performed for heart contrast enhanced. Same plans were copied and replaced to undo dose calculation errors that may be caused by CA. Eventually, dosimetric evaluations based on dose volume histograms (DVHs) of planning target volumes (PTV) and organs at-risk were studied and analyzed using the Wilcoxon's signed rank test. There is no statistically significant difference in dose calculation for the PTV maximum, mean, minimum doses, spinal cord maximum doses and lung volumes that received 20 and 30 Gy, between planes calculated with and without contrast scans (p > 0.05) and also for contrast scan, with manual regions overriding. Dose difference caused by the contrast agent is negligible and not significant. Therefore, there is no justification to perform two scans, and using an IV contrast enhanced scan for dose calculation is sufficient.

Dose calculation errors as a component of failing IROC lung and spine phantom irradiations.

Between July 2013 and August 2019, 22% of the imaging and radiation oncology core (IROC) spine, and 15% of the moving lung phantom irradiations have failed to meet established acceptability criteria. The spine phantom simulates a highly modulated stereotactic body radiation therapy (SBRT) case, whereas the lung phantom represents a low-to-none modulation moving target case. In this study, we assessed the contribution of dose calculation errors to these phantom results and evaluated their effects on failure rates. We evaluated dose calculation errors by comparing the calculation accuracy of various institutions' treatment planning systems (TPSs) vs IROC-Houston's previously established independent dose recalculation system (DRS). Each calculation was compared with the measured dose actually delivered to the phantom; cases in which the recalculation was more accurate were interpreted as a deficiency in the institution's TPS. A total of 258 phantom irradiation plans (172 lung and 86 spine) were recomputed. Overall, the DRS performed better than the TPSs in 47% of the spine phantom cases. However, the DRS was more accurate in 93% of failing spine phantom cases (with an average improvement of 2.35%), indicating a deficiency in the institution's treatment planning system. Deficiencies in dose calculation accounted for 60% of the overall discrepancy between measured and planned doses among spine phantoms. In contrast, lung phantom DRS calculations were more accurate in only 35% and 42% of all and failing lung phantom cases respectively, indicating that dose calculation errors were not substantially present. These errors accounted for only 30% of the overall discrepancy between measured and planned doses. Dose calculation errors are common and substantial in IROC spine phantom irradiations, highlighting a major failure mode in this phantom and in clinical treatment management of these cases. In contrast, dose calculation accuracy had only a minimal contribution to failing lung phantom results, indicating that other failure modes drive problems with this phantom and similar clinical treatments.

Impact of radiopacified bone cement on radiotherapy dose calculation.

Background and purposeRadiopacifiers are introduced to bone cements to provide the appearance of bone in kilovoltage (kV) radiographic images. For higher energy megavoltage (MV) radiotherapy treatment beams, however, these radiopacifiers do not cause a bone-like perturbation of dose. This study therefore aimed to determine the impact of the barium-contrasted plastic-based cement materials on radiotherapy dose calculations.Materials and methodsThe radiological properties of a physical sample of bone cement were characterised by computed tomography (CT) imaging and transmission measurements. Monte Carlo simulations of percentage depth-dose profiles were performed to determine the possible dose error for MV treatment beams. Dose differences were then investigated for clinical volumetric modulated radiotherapy treatment plans, with and without density overrides applied.ResultsDifferences of up to 7% were observed at the downstream interface of a 0.6 cm thick bone cement layer, compared to bone. Differences in planning target volume dose-volume metrics varied between −0.5% and 2.0%.ConclusionBefore planning radiotherapy treatments for patients who have undergone cranioplasty, every effort should be made to identify whether a radiopacified bone cement has been implanted. Density overrides should be applied to minimise dose calculation errors, whenever bone cement is used.

A Prospective Observational Study of Drug Therapy Problems in Pediatric Ward of a Referral Hospital, Northeastern Ethiopia.

Background A drug therapy problem is any undesirable event experienced by a patient during drug therapy that interferes with achieving the desired goals of therapy. It has been pointed out that hospitalized pediatric patients are particularly prone to drug-related problems. Identifying drug therapy problems enables risk quantification and determination of the potential impact of prevention strategies. The purpose of this study was to assess the drug therapy problems in a pediatric ward of Dessie Referral Hospital, northeast of Ethiopia, and to identify associated factors for drug therapy problems. Methods A prospective observational study design was carried out to assess drug therapy problems in a pediatric ward of Dessie Referral Hospital from February 1, 2018, to May 30, 2018. Ethical approval was obtained, and informed consent was signed by each study participant's parent before the commencement of the study. All patients admitted to the ward during the study period were included in the study. Data was collected by trained pharmacy staffs through medical record reviews of patients using a prepared standard checklist and semistructured questionnaire. The collected data were cleared and checked every day for completeness and consistency before processing. Data were entered, and descriptive statistical analysis was done using SPSS Version 20 Software. A P value of less than 0.05 was considered significant. Results The participants' mean age was 2.32 years with the standard deviation (SD) of 0.76 years. Among 81 patients, 71 (87.7%) of them had at least one drug therapy problem per patient which indicates that the prevalence of the drug therapy problem was substantially high. Needs additional drug was the most predominantly encountered drug therapy problem accounted (30 (25.2%)). On the other hand, ineffective drug was the least (3 (2.5%)) drug therapy problem. Antibiotics (47 (39.5%)) followed by fluid and electrolyte (25 (21%)) were classes of drugs mostly involved in the drug therapy problem. The main risk factors reported to the occurrence of the drug therapy problems were prescribing and dose calculation errors. Conclusion The present study revealed that majority of the patients had at least one DTP per patient; this indicates that prevalence of DTP was very high in the study area. Needs additional drug therapy followed by noncompliance was the major cause of the occurrence of DTP. Antibiotics were the main class of drugs involved in the drug therapy problem, and among the risk factors assessed, prescribing and dose calculation errors showed statistical significance.

Effects of different CT value assignment methods on dose calculations in radiotherapy planning for brain metastases.

BackgroundTo study the effects of different CT value assignment methods on the dose calculations in radiotherapy plans for brain metastases, this study will provide a reference for radiotherapy planning design based on MR images.MethodsAll fifty recruited patients underwent CT and MR simulated localization the same day as, but prior to, three dimensional conformal radiation therapy (3D-CRT) or intensity modulated radiation therapy (IMRT) for brain metastases. After rigid registration of both the CT and MR images, the main tissues and organs were delineated on the CT and MR images. The average CT value of each tissue or organ was calculated. Three groups of pseudo-CT were generated by three CT value assignment methods: (I) the whole tissue was assigned 140 HU; (II) cavity, bone and other tissues were assigned −700, 700 and 20 HU, respectively; (III) tissue- and organ-specific CT values were given. The dose distribution was recalculated based on the three groups of pseudo-CT to obtain Plan2, Plan3 and Plan4, accordingly. The resultant radiotherapy plans were considered the original plan (Plan1). Then, the dosimetric differences between these three plans and Plan1 were compared.ResultsThe average pseudo-CT values of bone and cavity were 731.7±69.3 and −725.5±26.1 HU, respectively. The range of average soft-tissue CT values was from −70 to 70 HU. The dose distribution between Plan1 and Plan2, Plan3 or Plan4 showed some differences, and the differences decreased in turn. The differences in the maximum dose of the lenses can reach 5.0%, 1.5% and 1.2%, respectively, while the differences in other dose parameters (maximum dose, mean dose and D98% to the PTV, D5% of the brainstem, and maximum dose of the brainstem, corpus callosum, left eye, right eye) were basically less than 2.0%, 1.2% and 0.8%, respectively. This shows that in the CT value assignment method, the dose calculation error can be greatly reduced by assigning the value to the bone and cavity separately, and if the different soft tissues are distinguished, the error of the dose calculation can be further reduced by more than 30%. In the pixel-by-pixel dosimetric comparison, the areas of more than 1% dose difference between Plan1 and Plan3 as well as Plan4 were mainly distributed near skin while those between Plan1 and Plan2 were mainly distributed at the bone, cavity, bone and soft tissues junction, and the skin near the field.ConclusionsIn summary, a scheme for assigning specific CT values to MRI-based radiotherapy is established. The scheme will provide patients with a dose-free radiotherapy plan. Through the calculation of the differences between the new plans and the old plan, it is found that our scheme can basically control the dose error below 0.8% to meet the clinical requirements.

Nuclear-interaction correction for patient dose calculations in treatment planning of helium-, carbon-, oxygen-, and neon-ion beams

In charged-particle therapy treatment planning, the patient is conventionally modeled as variable-density water, i.e. stopping effective density ρS, and the planar integrated dose distribution measured in water (PID) is applied for patient dose calculation based on path length scaling with the ρS. This approximation assures the range accuracy of charged-particle beams. However, it causes dose calculation errors due to water nonequivalence of body tissues in nuclear interactions originating from compositional differences. We had previously proposed and validated a PID correction method for the errors in carbon-ion radiotherapy. In the present study, we verify the PID correction method for helium-, oxygen-, and neon-ion beams. The one-to-one relationships between ρS and the nuclear effective density ρN of body tissues were constructed for helium-, carbon-, oxygen-, and neon-ion beams, and were used to correct the PIDs to account for the dose calculation errors in patient. The correction method was tested for non-water materials with un-scanned and scanned ion beams. In un-scanned beams penetrating the materials, the dose calculation errors of up to 5.9% were observed at the Bragg peak region, while they were reduced to ⩽0.9% by the PID correction method. In scanned beams penetrating olive oil, the dose calculation errors of up to 2.7% averaged over the spread-out Bragg peak were observed, while they were reduced to ⩽0.4% by the correction method. To investigate the influence of water nonequivalence of body tissues on tumor dose, we carried out a treatment planning study for prostate and uterine cases. The tumor over-doses of 0.9%, 1.8%, 2.0%, and 2.2% were observed in the uterine case for the helium-, carbon-, oxygen-, and neon-ion beams, respectively. These dose errors could be diminished by the PID correction method. The present results verify that the PID correction method is simple, practical, and accurate for treatment planning of these four ion species.

Comparison of the 2-D Dose Distribution Calculated by Planning System and Measured by Gafchromic Film Physical Dosimetry for 60Co and 192Ir Brachytherapy Sources.

Background: High Dose Rate (HDR) brachytherapy sources with high photon energy have been widely used in treating tumors. Dosimetric parameter of these brachytherapy sources should be determined according to the AAPM TG-43 recommendation. Gafchoromic films are reliable tools for this evaluation.Objective: The aim of this study is to evaluate and compare dose accuracy of the two-brachytherapy sources in a dedicated phantom.Material and Methods: In this analytical study, two common sources, including Cobalt and Iridium, were loaded into the dedicated phantom. The two-dimensional dose distribution around the source was calculated by TPS system for certain activities and geometries around the sources. Then, the experimental dose measured by Gafchromic film dosimetry was reported for different angles ranging from 0 to 180 degrees.Results: The difference between calculated and measured doses was less than 6 percent (-5 to +6 percent) for all of the channels and angles. These errors are smaller and mainly more than zero (Dfilm>DTPS) for angles less than 20 and larger than 110 degrees. There is no statistically significant discrepancy in dose calculation by treatment planning system. Conclusion: Although the estimated error in dose calculation is not significant, there is still an opportunity to increase the treatment precision. The correlation between the error and the angle should be considered in further plans of brachytherapy. The present study showed comparable errors compared to results of other research studies.

A sliding-window approach for improved VMAT dose calculation accuracy

The continuous delivery of volumetric modulated arc therapy (VMAT) plans is usually approximated by discrete apertures at evenly-spaced gantry angles for dose calculation purposes. This approximation can potentially lead to large dose calculation errors if the gantry angle spacings are large and/or there are large changes in the MLC apertures from one control point (CP) to the next. In this work, we developed a sliding-window (SW) method to improve VMAT dose calculation accuracy. For any 2 adjacent VMAT CPs ni and ni + 1, the dose distribution was approximated by a 2-CP SW IMRT beam with the starting MLC positions at CP ni and ending MLC positions at CP ni + 1, with the gantry angle fixed in the middle of the 2 VMAT CPs. Therefore, a VMAT beam with N CPs was approximated by a SW plan with N-1 SW beams. To validate the method, VMAT plans were generated for 10 patients in Pinnacle using 4° gantry spacing. Each plan was converted to a SW plan and dose was recalculated. Another VMAT plan, with 1° gantry spacing, was created by interpolating the original VMAT beam. The original plans were delivered on an Elekta Versa HD and measured with ArcCHECK. For both the isodose distribution and DVH, there were significant differences between the original VMAT plan and either the SW or the interpolated plan. However, they were indistinguishable between the SW and the interpolated plans. When compared with measurement, the average passing rates of the original VMAT plans were 87.3 ± 2.8% and 93.1 ± 1.0% for the 5 HN and 5 spine SBRT cases, respectively. On the other hand, the passing rates for both the VMAT1 and SW plans were above 95% for all the 10 cases studied. The dose calculation times of the original VMAT plans and the SW plans were very similar. We conclude that the proposed SW approach improves VMAT dose calculation accuracy without increase in dose calculation time.

An Empirical Model for Describing the Small Field Penumbra in Radiation Therapy.

Purpose We developed a mathematic empirical model for describing the small field penumbra in order to analyze the potential dose perturbation caused by overlapping field to avoid the dose calculation errors in linear accelerator-based radiosurgery. Materials and methods A ball phantom was fabricated for measuring penumbra at 4 different gantry angles in the coplanar plane. A least square root estimation (LSRE) Model was created to fit the measured penumbra dose profile and to predict the penumbra dose profile at any gantry angles. The Sum of Squared Errors (SSE) was used for finding the parameters n and t for the best fitting of the LSRE model. Geometric and mathematical methods were used to derive the model parameters. Results The results showed that the larger the gantry angle of the field, the more the expansion of the penumbra dose profile. The least square root estimation model for describing small field penumbra is as follows: where PenumbraD(š) denotes the dose profile D(š) at the penumbra region, T is the penumbra height (usually in scalar 100), n is the parameter for curvature, š = x − W d/2 (x and š are the values in cm on x-axis), and t is the radiation transmission of the collimator. Geometric analysis establishes the correlation between the penetration depth of the exposure and its effect on the penumbra region in ball phantom. The penumbra caused by an exposure at any arbitrary angles can be geometrically derived by using a one-variable quadratic equation. Conclusion The dose distribution in penumbra region of small field can be created by the LSRE model and the potential overdosage or underdosage owing to overlapping field perturbation can be estimated.

PO-0900 The magnitude of dose calculation errors as a component of IROC phantom failures

PO-0900 The magnitude of dose calculation errors as a component of IROC phantom failures

Verification of the Elekta Monaco TPS Monte Carlo in modelling radiation transmission through metals in a water equivalent phantom.

Many studies have performed dosimetric studies using various metal implants however these are difficult to translate to other implants of a different geometry or material (Rijken and Colyer, J Appl Clin Med Phys 18:5:301-306, 2017; Ade and du Plessis, J Appl Clin Med Phys 18:5:162-173, 2017; Prabhakar et al. Rep Pract Oncol Radiother 18:209-213, 2013; Ng et al. Rep Pract Oncol Radiother 20:273-277, 2015; Reft et al. Med Phys 30:1162-1182, 2003; Sasaki et al., Nihon Hoshasen Gijutsu Gakkai Zasshi 72(9):735-745, 2016). In this study, the ability of the Monaco Monte Carlo algorithm (Elekta AB, Stockholm, Sweden) to model radiation transport through different types of metals was evaluated. Investigation of the capabilities and limitations of the algorithm is required for the potential use of Monaco for planning radiotherapy treatments when avoidance of metal implants is clinically undesirable. A MapCHECK 2 diode array (Sun Nuclear Corp, Melbourne, USA) and a PTW 30013 Farmer chamber was used to measure the dose at depth, downstream of 1cm × 5cm × 5cm metal blocks of three known compositions; stainless steel, aluminium and MCP96. The setup was imaged using a CT scanner and imported into the Monaco TPS where the beam arrangement was replicated. The density of the metals was overridden using the known electron density of each (IMPAC Medical Systems Inc, Monaco dose calculation technical reference. IMPAC Medical Systems, Sunnydale, CA, 2013). The differences between the dose measured using the ion chamber and calculated using Monaco downstream of the 1cm metal blocks were respectively: - 1.2%, - 2.2% and 9.5% when irradiated using a 6 MV beam, and - 0.9%, - 1.3% and 14%, when irradiated using a 15 MV beam. This was then repeated using 2cm and 3cm of each metal type giving similar results for aluminium and stainless steel and increased discrepancy for MCP96. Discrepancies between treatment planning software and measurements at depth have been shown to give uncertainties between 5 and 23% in previous studies (Rijken and Colyer, J Appl Clin Med Phys 18:5:301-306, 2017; Ade and du Plessis, J Appl Clin Med Phys 18:5:162-173, 2017; Prabhakar et al. Rep Pract Oncol Radiother 18:209-213, 2013; Ng et al. Rep Pract Oncol Radiother 20:273-277, 2015; Reft et al. Med Phys 30:1162-1182, 2003; Sasaki et al., Nihon Hoshasen Gijutsu Gakkai Zasshi 72(9):735-745, 2016). This study uses basic shapes providing results that remove the uncertainties in geometry and can therefore be applied to any shape. This will help determine whether errors in dose calculations are due to the TPS particle transport algorithms or due to other effects, such as inaccurate contouring or incorrect densities. Thus giving the planner an additional degree of freedom in their planning and decision making process.

Dose calculation deviations induced by fractional image-guided-couch-shifts for Varian Halcyon MV cone beam CT

PurposeTo assess the criticality of calculation deviations induced by fractional image-guided-couch-shifts for Halcyon MV cone beam CT (CBCT) dose, which is incorporated as part of total treatment dose. MethodsEclipse-calculated imaging dose was first validated in ‘Cheese Phantom’ by measurement. Then, the actual imaging dose (Dact) for 18 historical patients of various sites were recalculated based on 513 MV CBCT-guided-couch-shift data, and compared with reference computations based on treatment isocentre (Dref). Patient- and plan-specific dose from treatment fields was integrated with Dact and Dref respectively for comparison. ResultsThe average absolute relative disagreements between the measured and calculated dose were less than 1.23%. The mean ± 1SD of gamma passing rates of the accumulated imaging dose and total dose were 80.71 ± 6.22% and 99.81 ± 0.32% respectively based on 3 mm/3%/local/10% threshold criteria. The accumulated errors of minimum imaging dose to PTV were no larger than −14.38 cGy, which were reduced to −0.82 cGy after the heterogeneous treatment dose was overlaid. The mean relative discrepancies of PTV minimum dose were −0.61 cGy (−0.71%) and −0.00 (0.00%), before and after incorporating the treatment dose respectively. ConclusionsThe Eclipse-calculated Halcyon MV CBCT dose was validated. Although the isocentre displacement-induced imaging dose calculation errors for Halcyon MV CBCT were partially cancelled out by couch shifts of various directions and distances, especially after the incorporation of heterogeneous treatment dose, it was still advisable to monitor the accumulated deviations and replan when unacceptable target under-dose or organ over-dose were observed.

Evaluation of the accuracy of calculated dose with the EPL and ETAR algorithms using thorax CIRS phantom.

Introduction: Each treatment planning algorithm has some errors in dose calculation. Particularly these errors are more pronounced in the heterogeneous regions. It is recommended for each set of radiation data and algorithm subtle deliberation done regarding dose calculation accuracy. Knowing the amount of error in dose calculation will result in a fairly accurate estimate of the actual absorbed dose to the tumor. Also, if the errors have a certain trend, the correction coefficients can be applied to emend the dose. There are limited reports that have examined the algorithm in their health center. This study aimed to evaluate the EPL and ETAR algorithms in radiation dose calculation using thorax CIRS phantom. Materials and Methods: In this empirical study TEC-DOC 1583 guideline provided by IAEA was used. Experimental measurements were obtained using an ionization chamber at the recommended points according to above instructions for 6 and 18 MV energies. The values of the measurements were compared with the values obtained from the calculation of the EPL and ETAR algorithms using acceptance criteria specified in the instruction. A total of 31 measurements and calculations were obtained for each algorithm with a specific energy. Due to the small number of measurable points in the phantom, for each test, difference between the measurement and calculation was normalized to the reference point of each test. Results: In the EPL algorithm for 6 MV energy, the calculations were in agreement with measurements for 27 dose points and differences between measurements and calculations ranged from 0.1% to 10.4%. For 18 MV energy, the calculations were in agreement with measurements for 21 dose points and differences between measurements and calculation ranged from 0.4% to 13%. In ETAR algorithm for 6 MV, calculations were in agreement with measurements for 21 dose points, and differences between measurements and calculations ranged from 0.1% to 9%. For 18 MV, the calculations were in agreement with measurements for 17 dose points and differences between calculations and measurements ranged from 0% to 11%. Conclusion: For the EPL algorithm, compared with the ETAR algorithm, more dose points were agreed with acceptance criteria. Although the numerical values of the errors in the ETAR algorithm were 1 to 2 percent less than the EPL algorithm. The greatest error occurs when calculations made in low density lung tissue with inhomogeneities or in high density bone. Errors were larger in shallow depths. Overall, the error in higher energy was more than low energy beam.

27 CBCT-based dose calculation for head-and-neck tumors treated with VMAT in order to approach adaptive radiotherapy

Introduction Head-and-neck (H&N) treatment delivery time is divided in 35 fractions of 2 Gy and takes 7 weeks. Anatomical modifications can occur during treatment and introduce uncertainties in VMAT dose distribution. Cone Beam Computed Tomography (CBCT) is used for patients positioning but might be considered for dose calculation. The purpose of this study is to improve the image quality and accuracy of CBCT-based dose calculation through the use of an optimized CBCT acquisition mode to finally build a patient-specific H&N deformation model to approach adaptive radiotherapy (ART). Methods A calibration was performed with the view to achieve a higher accuracy in a 25 cm field-of-view (FOV) CBCT ORL , ART (Varian Clinac® OBI 1.6) image-based dose assessment by varying the source-to-imager distance (SID), tube voltage (kVp) and mAs. Then, the Hounsfield unit (HU) vs relative electron density curves were drawn for the CIRS (062 M) phantom associated to the QuasarTM (Modus QA) using the post-treatment software Syngo.via (Siemens Healthinners) with the intention of comparing them to the planning CT curve ( CT plan ) . The optimized CBCT ORL , ART acquisition mode dose has been assessed as described in the AAPM report n. 111 [1] with the use of a 0.3 cm3 ionization chamber inside a 26 cm PMMA cylindrical phantom. This retrospective cohort study includes 38 H&N cancer patient cases who underwent VMAT treatment with one CBCT ORL , ART per week in 2017. For each patient, a deformable image registration algorithm (DIR) has been applied on the CT plan and on the 7 CBCT ORL , ART using Syngo.via. A comparative study between deformation vector fields (DVF), planned target volume (PTV) and organ-at-risk (OAR) contours similarity measurements and dose-volume histogram (DVH) has been done with the aim of assessing the feasibility of CBCT-based dose calculation. Results The optimized CBCT ORL , ART mode chosen had the following acquisition parameters: 100 kVp, 145 mAs and a 150 cm SID. A 3.83 mGy CTDIw has been assessed for the CBCT ORL , ART mode with a + 1.8% relative gap between measured and constructor data. Due to the limited field of view (FOV), truncation of the CBCT caused incorrect deformations when the DIR algorithm was applied and led to significant errors in dose calculation while deforming the CT plan image to match the CBCT ORL , ART was considered suitable for dose calculation based on the 2 mm gamma-test on over 95% of the voxels and DVH. Conclusions This first adaptive radiotherapy approach verifies the feasibility of CBCT-based dose calculation. Following this study, a dosimetric alert protocol was set up to evaluate the need of performing a re-plan during the treatment.

Murine vs human tissue compositions: implications of using human tissue compositions for photon energy absorption in mice.

Dual energy CT (DECT) images of 9 female mice were used to extract the effective atomic number Zeff and the relative electron density ρe for each voxel in the images. To investigate the influence of the tissue compositions on the absorbed radiation dose for a typical kilovoltage photon beam, mass energy-absorption coefficients μen/ρ were calculated for 10 different tissues in each mouse. Differences between human and murine tissue compositions can lead to errors around 7.5 % for soft tissues and 20.1 % for bone tissues in μen/ρ values for kilovoltage photon beams. When considering the spread within tissues, these errors can increase up to 17.5 % for soft tissues and 53.9 % for bone tissues within only a single standard deviation away from the mean tissue value. This study illustrates the need for murine reference tissue data. However, assigning only a single mean reference value to an entire tissue can still lead to large errors in dose calculations given the large spread within tissues of μen/ρ values found in this study. Therefore, new methods such as DECT and spectral CT imaging need to be explored, which can be important next steps in improving tissue assignment for dose calculations in small animal radiotherapy. This is the first study that investigates the implications of using human tissue compositions for dose calculations in mice for kilovoltage photon beams.

Treatment plan complexity does not predict IROC Houston anthropomorphic head and neck phantom performance

Previous works indicate that intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) plans that are highly complex may produce more errors in dose calculation and treatment delivery. Multiple complexity metrics have been proposed and associated with IMRT QA results, but their relationships with plan performance using in situ dose measurements have not been thoroughly investigated. This study aimed to evaluate the relationships between IMRT treatment plan complexity and anthropomorphic phantom performance in order to assess the extent to which plan complexity is related to dosimetric performance in the IROC phantom credentialing program. Sixteen complexity metrics, including the modulation complexity score (MCS), several modulation indices, and total monitor units (MU) delivered, were evaluated for 343 head and neck phantom irradiations, comprising both IMRT (step-and-shoot and sliding window techniques) and VMAT. Spearman’s correlations were used to explore the relationship between complexity and plan performance, as measured by the dosimetric differences between the treatment planning system (TPS) and thermoluminescent dosimeter (TLD) measurement, as well as film gamma analysis. Relationships were likewise determined for several combinations of subpopulations, based on the linear accelerator model, TPS used, and delivery modality. Evaluation of the complexity metrics presented here yielded no significant relationships (p > 0.01, Bonferroni-corrected) and all correlations were weak (less than ±0.30). These results indicate that complexity metrics have limited predictive utility in assessing plan performance in multi-institutional comparisons of IMRT plans. Other factors affecting plan accuracy, such as dosimetric modeling or multileaf collimator (MLC) performance, should be investigated to determine a more probable cause for dose delivery errors.

Assessment of electron density effects on dose calculation and optimisation accuracy for nasopharynx, for MRI only treatment planning.

Computed tomography (CT) is the gold standard for radiotherapy simulation and treatment planning, providing spatial accuracy, bony anatomy definition and electron density information for dose calculations. Magnetic resonance imaging (MRI) has been introduced in radiotherapy to improve visualisation of anatomy for accurate target definition and contouring, however lacks electron density information required for dose calculations, with various methods used to overcome this. The aim of this work is to assess the impact on dose calculation accuracy and optimisation results of different approaches to determine electron density, as could be used in MRI only treatment planning for nasopharyngeal datasets with VMAT treatment plans. Volumetric modulated arc therapy (VMAT) plans were created for 10 retrospective head and neck (H&N) nasopharyngeal patients. The VMAT plans were generated on the gold standard dataset, the original CT scan. Data sets with no density correction (water equivalent) and two different sets of bulk density correction for bone/air/tissue applied separately were generated for these patients and the VMAT plans were recalculated for each case. Plans were also reoptimised on these data sets, and recalculated. Optimisation error was assessed through equivalent uniform dose (EUD) differences. Additionally, point dose comparison, dose volume histogram (DVH) analysis and gamma analysis of dose were used to assess dose calculation error. The dose calculation error on average was an increase in EUD whereas the optimisation error on average was a reduction in EUD compared to the original plan for all datasets aside from the bone only override dataset where bone was set to 1.61g/cm3. For the optimisation error, the largest mean absolute error (MAE) was 1.88Gy EUD for the PTV, and 2.21Gy EUD for the brainstem, for the reoptimisation completed on the air only overridden dataset, and recalculated on the original. Bulk density corrections for bone and air provide dose calculations within 3% of the original treatment plans. Optimisation errors have the potential to be greater than dose calculation errors if incorrect density corrections are utilized. Electron density correction using a bulk density approach achieves dose calculation uncertainties within 3%, however more advanced approaches, such as a voxel based approach, may improve accuracy and should be considered.